Optical modulators, such as high-speed optical modulators, are useful in photonic systems. High speed optical modulators may operate pursuant to principals of electrorefraction (ER) or electroabsorption (EA). ER is often used for amplitude and phase modulation, while EA is used for amplitude modulation. An advantage provided by electrorefraction, as compared to electroabsorption, is the maintenance of low linear loss. Low optical absorption leads to low electron-hole pair generation and low heat generation. Accordingly, electrorefractive modulators can generally handle higher optical power densities than electroabsorptive modulators, due, in part, to the absence of bleaching and overheating. High optical power densities are desirable for radio-frequency (RF) photonic and long-haul optical fiber communication systems.
Additionally, low optical absorption allows for use of a Fabry-Perot or ring resonator to create “slow-wave” modulation. In slow wave modulation, modulation efficiency may be enhanced by orders of magnitude, due, in part, to enhanced interaction time. However, Fabry-Perot have conventionally failed to provide sufficient tunability for use in certain telecommunications applications.
Currently, commonly used materials for high-speed phase modulators include lithium niobate (LN), III–V compound semiconductors, and polymers. LN has been studied for commercial applications, such as acoustic-wave filters for mobile phones. Traveling-wave LN modulators have achieved bandwidths exceeding 100 GHz. However, LN modulators can generally not be integrated with III–V active components, due to material incompatibilities. Moreover, the low electrooptic coefficient of lithium niobate (r33˜30 pm/V) may lead to a high operating voltage (VπL=40 to 80 Vmm), and hence long device size and high power consumption. Nonetheless, monolithic integration would be desirable in that it would allow for realization of low-cost photonic subsystems with high functionality and speed.
A main advantage of polymeric modulators is the relatively easy fabrication processes associated with it. Further, polymeric modulators may provide a high modulation bandwidth exceeding 110 GHz. However, polymeric modulators exhibit low electrooptic coefficients in the 10 to 70 pm/V range, which, in combination with a low refractive index, generates sensitivity similar to LN (VπL=30 to 200 Vmm). Other drawbacks of polymeric modulators include unproven lifetime and stability, and limited maximum optical power density and operating temperature.
Phase modulators based on III–V semiconductors are generally advantageous since, unlike LN or EA based modulators, they can provide both monolithic integration and high optical saturation power. However, although optical modulators based on III–V quantum wells have a higher sensitivity (VπL=10 to 20 Vmm) as compared to LN and polymeric modulators, III–V quantum well optical modulators are typically difficult to couple to fiber optics, and hence are not generally suitable for discrete devices. Nonetheless, III–V modulators serve well for monolithic integration with other III–V active components.
Power consumption is important in an integrated device, since power generation and transfer, as well as heat dissipation, are significantly restricted in a closely packed integrated subsystem. Therefore, there have been significant efforts to enhance material sensitivity in modulators, since modulator power consumption is inversely proportional to the square of the material sensitivity. Theoretically, more than one order of magnitude enhancement of sensitivity has been predicted for symmetric and asymmetric coupled quantum wells. More specifically, one order of magnitude reduction in Vπ has been predicted using asymmetric coupled quantum wells in III–V based phase modulators. Experimental results however, have shown enhancements approaching a factor of five in a GaAs/AlGaAs material system. Most desirably, high-speed and high-power integrated modulators with Vπ<1 volt may provide for advanced RF-photonic fiber optic links. Unfortunately, such improvements, in accordance with these experimental results, have not yet been demonstrated for indium phosphide (InP)-based modulators, which are attractive for telecommunication applications.
Therefore, the need exists for a method and apparatus for realizing a low voltage InP-based modulator that lowers operating voltages and power consumption of devices employing the modulator, as compared to devices employing conventional modulators.